| Large-scale mechanical presses are high-end equipment and are important equipment for sheet metal forming.They have been widely used in automobiles,ships,and airplanes.Large mechanical presses have the characteristics of high technical content,difficulty in design and manufacture,and complex control systems.Their technical level reflects the advanced manufacturing level of a country.The hydraulic overload protection system is one of the important devices in the large mechanical press.If the pressure is overloaded,the unloading can be performed instantaneously to protect the press and avoids damage to the mold.It also has the function of automatic pressure compensation after unloading and restores automatic protection functions.Because the hydraulic overload protection system is a complex mutual coupling system composed of gas,liquid and solid three-phase medium,the coupling mechanism and response characteristics are not very clear.In the actual application process,the unloading control accuracy is often low.It is easy to cause the unloading problem when the pre-loading is not reached before the specified load state and the specified load is reached,resulting in the discontinuity of the normal production state and the damage of the equipment mold.This paper takes the hydraulic overload protection system of large mechanical press as the research object.Aiming at the problem that the hydraulic overload protection system does not reach the specified load state ahead of unloading and does not reach the specified load unloading problem,the systematic research was carried out by means of mechanical theory modeling.mathematical modeling and simulation of hydraulic system,two-way liquid-solid interaction analysis and experimental verification.The main research work is as follows:On the basis of system analysis of connector working process,the connector model is simplified and the equivalent spring model of the connector and the hydraulic pad is constructed.Theoretically,the effects of equivalent spring stiffness of connector,equivalent spring stiffness of hydraulic pad and initial pre-pressure of hydraulic pad on the connector's transmission force are studied respectively.The following conclusions are obtained:(1)The greater the unloading pressure of the unloading valve,the greater the transmission force of the connector;(2)When the unloading pressure of the unloading valve is constant,the smaller the pre-pressure of the hydraulic pad,the greater the transmission force of the connector;(3)The greater the equivalent spring rate coefficient of the connector,the greater the transmission force of the connector;(4)The smaller the equivalent spring stiffness coefficient of the hydraulic pad,the greater the transmission force of the connector.The finite element method is used to analyze the three-dimensional model of the connector.By applying different pre-pressure to the hydraulic pad and using the top surface of the piston as the displacement reference,the equivalent spring stiffness fitting formula of the connector is derived.Based on the systematic analysis of the factors affecting the volume elastic modulus of the hydraulic pad,a formula for calculating the equivalent spring stiffness of the hydraulic pad is derived;In order to verify the rationality of the model,ADAMS software is used to simulate the equivalent spring model of connector.By applying different press working forces to construct different working conditions,it is concluded that the elastic restoring force of the equivalent spring of the connector accounts for about 8%of the working force of the press.That is,when calculating the static unloading pressure of the hydraulic overload protection system,the overshoot of liquid pressure should be 108%of the working pressure of the press,and the empirical value currently used is generally recommended to be 110%.Therefore,from the point of view of static load analysis,the analysis model has a good agreement with the engineering experience value.It is verified that the equivalent spring analysis model built for connector and calculation method are correct and have high reference value for engineering applicationThe influence of hydraulic pad pre-pressure,working force,elastic modulus of pipe material,pipe length,pipe diameter,pipe wall thickness and other variables on oil pressure of large mechanical press were studied.Using the analysis idea from partial to whole,firstly establish a mathematical model only for pipeline analysis.Through the analysis,the influences of the hydraulic pad pre-pressure,the working force of the press,the bulk modulus of the pipe material,the length of the pipe,the diameter of the pipe,the wall thickness of the pipe,etc.on the effective bulk modulus of the liquid-filled pipeline and the oil pressure of the pipeline are obtained.On this basis,an integrated analysis model including hydraulic cushion filling process and press working force application process is constructed.Using univariate analysis method,the influence of pre-pressure of hydraulic pad,length of pipe,diameter of pipe,wall thickness of pipe,equivalent spring stiffness of connector and other variables on the pressure change of liquid-filled pipe was analyzed,the conclusions are obtained as follows:(1)The smaller the initial pressure of the hydraulic pad,the smaller the overshoot of the oil pressure in the pipeline relative to the working pressure of the press;(2)The longer the pipe length,the lower the oil pressure in the pipe;(3)The larger the pipe diameter,the lower the oil pressure in the pipe;(4)The wall thickness has little effect on the change of oil pressure:(5)The greater the equivalent spring stiffness of the connector,the lower the oil pressure in the pipeline.The research results provide theoretical support for engineering applicationBy constructing the center combined test design scheme,a total of 60 sample data points was used to calculate the Pareto diagram.The influence law and sensitivity of each parameter on the oil pressure in the filling pipeline are summarized,and the following conclusions are obtained:The length of the pipeline has the greatest influence on the change of oil pressure in the pipeline.The longer the pipeline,the lower the oil pressure in the pipeline;The second major impact is the hydraulic pad initial pre-pressure.The greater the pre-pressure,the greater the oil pressure in the pipeline;It is pipeline diameter again,namely pipeline diameter is bigger,the oil pressure in the pipeline is lower;The effect of the connector's equivalent spring stiffness coefficient is minimal,that is,the greater the connector's equivalent spring stiffness coefficient,the lower the oil pressure in the pipeline.The analysis results are consistent with the experimental results.80 sample points were obtained through the Latin hypercube test scheme,and the multi-parameter and three-parameter RSM(Response Surface Methodology)Response Surface approximation model was respectively constructed.The variation law of the oil pressure in the pipeline affected by each variable is consistent with the experimental conclusion,indicating that the established RSM approximation model has good engineering application value.The bidirectional liquid-solid coupling direct solution method is used to simulate the model of the connector and the unloading valve.In order to simplify the model and improve the calculation efficiency,a 2D bidirectional liquid-solid coupling analysis model was established.The research method solved the problem of dynamic mesh technology under the small displacement sliding between the piston and the cylinder wall,and the mesh reconstruction technology problem of the gap between the pilot valve spool and the valve seat.It is concluded that the slight movement of the piston xwill cause a sharp change in the liquid pressure,which easily breaks the stress balance state of the pilot valve spool,and then causes the main valve to open for pressure unloading.The liquid control equation and the structural control equation are combined into a control equation,and the direct method is used to solve the problem.It is proved that it is feasible to solve the bidirectional liquid-solid coupling model Imethod by direct method.The analysis conclusion provides theoretical support for the experimental data analysis of the direct connection scheme.The hydraulic overload protection system has the characteristics of closed liquid chamber,liquid micro-compressibility,small displacement and impact load.In order to systematically study the dynamic response characteristics of the liquid filled pipeline,a two-way liquid-solid coupling liquid-filled pipeline analysis model was established.The research method solved the problem that the bidirectional liquid-solid coupling of a closed liquid cavity is difficult to converge under the action of similar impact load.Through simulation analysis,it is concluded that:The mutual coupling of the liquid and the pipe will cause fluctuations in the liquid pressure in the pipe,causing the liquid pressure in the pipe to be greater than the working load pressure of the press,and a shock wave will be generated in the liquid.During the process of the press,the liquid pressure at different pipe positions is different,passing back and forth in a certain cycle.The value of the first pressure shock wave is the maximum,and then the liquid pressure gradually converges to stabilize.Under the impact load,the pipe wall will produce vibration phenomenon,and the vibration amplitude is the largest at the bend position.The bidirectional liquid-solid coupling analysis method has provided a technical reference for solving the bidirectional liquid-solid coupling problem with closed liquid cavity,small displacement,micro-compressible liquid and impact load characteristics.The effects of three variables of pipe length,initial pressure of hydraulic pad and working force of the press on the liquid pressure in the pipeline were studied by the test method.Three test schemes including unloading valve and connector adopting direct connection,lengthened pipe mode one and lengthened pipe mode two are designed.The liquid pressure response characteristics of the liquid-filled pipeline under different hydraulic pad pre-pressure and different press working forces were studied.Through the analysis and comparison,the variation law of liquid pressure in the pipeline was obtained,and the following conclusions were drawn:(1)No matter what the pre-pressure value of the hydraulic pad is,under the same working force of the press,the longer pipe can reduce the liquid pressure in the pipe.The liquid pressure,overshoot of liquid pressure and overshoot ratio of liquid pressure in the pipe with direct connection mode are the maximum;(2)Under the same working force of the press,different hydraulic pad pre-pressure and different pipe length will have different effects on the liquid pressure in the pipe;(3)Through experiments,it is found that when the hydraulic pad pre-pressure is 50%(125 bar)of the working pressure of the press.as the working force increases,the pipe force is gradually reduced,which may be affected by the frequency response characteristics of the pipe structure,and the working force of the press is excited.The frequency close to the resonance frequency of the pipeline causes the liquid pressure in the pipeline to be larger than other pre-pressure values,but as the working force increases,the diameter of the pipeline becomes larger,and the liquid pressure value in the pipeline decreases.The experimental data of the extended tube one is compared with the three-parameter RSM approximation model,it is verified that the test results are in good agreement with the simulation results,indicating that the three-parameter RSM approximation model has a high degree of confidence.By collating and analyzing the test data,the RSM approximation model is constructed respectively,which can help the enterprise to quickly obtain the coupling response relationship between hydraulic pad pre-pressure,press working force,pipe length and liquid pressure in the pipeline during the design process,improve the design efficiency. |
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